Change some names around

 - Shortening "Branding Guidelines" to "Brand Guidelines" for punchiness
 - Broadening "Data Model" to "System Architecture." More parallel to
   "Information Architecture," and informs things like
   https://github.com/gittip/www.gittip.com/pull/2006 and
   https://github.com/gittip/www.gittip.com/issues/1939.